Gas turbine blades operating in a hot environment often are cooled with air passing from inside the blade out through cooling air holes. A multiplicity of these cooling air holes are drilled through the material of the blade at closely spaced locations. Typically there can be 400 such holes in a single blade.
A known method of drilling these holes uses a pulsed laser beam. In accordance with such procedure a high power beam is focused on the material at a location where the hole is to be drilled. The laser is repetitively pulsed until the hole is drilled through the material. In such a manner, 0.6 mm holes can be drilled through a 2 mm thickness of 5544 waspalloy (a nickel based alloy) using 630 microseconds pulses. Each pulse has an energy level of 6 joules with a pulse rate being on the order of 5 Hz. After completion of a single hole the beam is refocused at the next location to drill the next hole.
With a large number of holes being drilled it is desirable to increase the speed of drilling. The laser is capable of operation at a higher frequency. However, effective drilling cannot be carried out at these higher rates. At 5 Hz the hole is drilled in 10 pulses, while increasing the rate to 50 Hz requires 15 pulses to accomplish the same drilling.
This additional energy goes into heating of the material. This raises the temperature of the bounding wall of the hole and increases the size of the heat affected zone. The rapid rate also provides less time for cooling between pulses.
Since the laser beam is being focused from a lens and has a shallow conical shape, the focus at the surface will not provide a precise focus at deeper locations. In practice, the beam striking the edge of the partially drilled hole at an acute angle reflects it down the hole where it impinges at the bottom, melting and vaporizing material. The higher temperature of the bounding walls makes them less reflective thereby further decreasing the efficiency of the operation. This also increases the chance of erratic sizing along the length of the hole.